Polyethylene composition for pressure pipes with enhanced flexibility

ABSTRACT

The present invention relates to a polyethylene composition comprising a base resin which comprises (a) an ethylene homo- or copolymer fraction (A); and (b) an ethylene homo- or copolymer fraction (B), wherein (i) fraction (A) has a lower average molecular weight than fraction (B); (ii) the base resin has a density of 940 to 947 kg/m 3 ; (iii) the polyethylene composition has an MFR 5  of 0.1 to 0.5 g/10 min; and (iv) the polyethylene composition has an SHI (2.7/210)  of 10 to 49. Furthermore, the present invention relates to an article, preferably a pipe comprising said composition and to the use of said composition for the production of an article, preferably a pipe.

The present invention relates to a polyethylene composition for pipeswhich comprises a polymeric base resin comprising two polyethylenefractions with different molecular weight. Furthermore, the presentinvention relates to an article, preferably a pipe, comprising saidcomposition and to the use of said composition for the production of anarticle, preferably of a pipe.

Polyethylene compositions comprising two or more polyethylene fractionswith different molecular weight are often referred to as bimodal ormultimodal polyethylene compositions. Such polyethylene compositions arefrequently used e.g. for the production of pipes due to their favourablephysical and chemical properties as e.g. mechanical strength, corrosionresistance and long-term stability. When considering that the fluids,such as water or natural gas, transported in a pipe often arepressurized and have varying temperatures, usually within a range of 0°C. to 50° C., it is obvious that the polyethylene composition used forpipes must meet demanding requirements. On the other hand, to facilitateinstallation of the pipes e.g. into the ground, a high flexibility ofthe pipes is desired.

In particular, the polyethylene composition used for a pipe should havehigh mechanical strength, good long-term stability, notch/creepresistance and crack propagation resistance, and, at the same time highflexibility. However, at least some of these properties are contrary toeach other so that it is difficult to provide a composition for pipeswhich excels in all of these properties simultaneously. For example,stiffness imparting mechanical strength to the pipe is known to improvewith higher density but, in contrast, flexibility and notch/creepresistance is known to improve with reduced density.

Furthermore, as polymer pipes generally are manufactured by extrusion,or, to a smaller extent, by injection moulding, the polyethylenecomposition also must have good processability.

It is known that in order to comply with the contrary requirements for apipe material, bimodal polyethylene compositions may be used. Suchcompositions are described e.g. in EP 0 739 937 and WO 02/102891. Thebimodal polyethylene compositions described in these documents usuallycomprise two polyethylene fractions, wherein one of these two fractionshas a lower molecular weight than the other fraction and is preferably ahomopolymer, the other fraction with higher molecular weight preferablybeing an ethylene copolymer comprising one or more alpha-olefincomonomers.

One great disadvantage of such pipes when used for gas or cold waterinfrastructure is the lack of flexibility of the pipes. The pipes arerigid and strong. These mechanical properties are the result of the highdemands regarding mechanical strength and long-term stability.

In laying known gas or cold water pipes, for example in open-trenchlaying or trenchless laying technologies like plough-in-place laying,often problems occur due to the stiffness of the pipes. It is oftendifficult to align and manoeuvre the pipes into the trenches. Stillfurther, it is often a problem to straighten pipes which are stored ortransported as coils. The same problem occurs if bends have to be passedwhich is particularly important for pipes of smaller and medium size.All these problems are of course even more relevant when the stiffnessof the pipes increases due to lower temperature, for example in coldweather.

It is thus particularly desirable to provide a pipe with enhancedflexibility without loosing the mechanical strength and the long termstability.

Accordingly, it is the object of the present invention to provide apolyethylene composition for pipes having an improved combination ofproperties, in particular having enhanced flexibility and,simultaneously, high mechanical strength and good long-term stability.

The present invention is based on the surprising finding that the abovementioned object can be achieved by a polyethylene compositioncomprising at least two polymer fractions with different molecularweights, having carefully selected values of density and MFR₅ withinsmall ranges and the polyethylene composition having a rather low SHI.

Accordingly, the present invention provides a polyethylene compositioncomprising a base resin which comprises

-   -   (a) an ethylene homo- or copolymer fraction (A); and    -   (b) an ethylene homo- or copolymer fraction (B), wherein    -   (i) fraction (A) has a lower average molecular weight than        fraction (B);    -   (ii) the base resin has a density of 940 to 947 kg/m³;    -   (iii) the polyethylene composition has an MFR₅ of 0.1 to 0.5        g/10 min; and    -   (iv) the polyethylene composition has an SHI_((2.7/210)) of 10        to 49.

It has been found that with such polyethylene compositions pipes can beproduced which have an enhanced flexibility. Therefore, pipes made ofthe inventive polyethylene composition can more easily be straightened,aligned into the trenches and passed around corners. Nevertheless, suchpipes have also high mechanical strength, which e.g. allows for the pipebeing used for the transport of pressurized fluids, an excellentlong-term stability and a good rapid crack propagation resistance.Furthermore, the polyethylene compositions also have goodprocessability.

It should be noted that the composition of the present invention ischaracterised not by any single one of the above defined features, butby their combination. By this unique combination of features it ispossible to obtain pipes of superior performance, particularly withregard to flexibility and rapid crack propagation (RCP), while minimumrequired strength (MRS), processability, impact strength and slow crackpropagation resistance are maintained.

The term molecular weight where used herein denotes the weight averagemolecular weight M_(w).

The term “base resin” denotes the entirety of polymeric components inthe polyethylene composition according to the invention, usually makingup at least 90 wt % of the total composition. Preferably, the base resinis consisting of fractions (A) and (B), optionally further comprising aprepolymer fraction in an amount of up to 20 wt %, preferably up to 10wt %, more preferably up to 5 wt % of the total base resin.

In addition to the base resin, usual additives for utilization withpolyolefins, such as pigments, stabilizers (antioxidant agents),antacids and/or anti-UVs, antistatic agents and utilization agents (suchas processing aid agents) may be present in the polyethylenecomposition. Preferably, the amount of these additives is 10 wt % orbelow, further preferred 8 wt % or below, still more preferred 4 wt % orbelow of the total composition.

Preferably, the composition comprises carbon black in an amount of 8 wt% or below, further preferred of 1 to 4 wt %, of the total composition.

Further preferred, the amount of additives different from carbon blackis 1.5 wt % or less, more preferably 1.0 wt % or less, most preferably0.5 wt % or less.

Usually, a polyethylene composition such as that of the presentinvention, comprising at least two polyethylene fractions, which havebeen produced under different polymerisation conditions resulting indifferent weight average molecular weights for the fractions, isreferred to as “multimodal”. The prefix “multi” relates to the number ofdifferent polymer fractions the composition is consisting of. Thus, forexample, a composition consisting of two fractions only is called“bimodal”.

The form of the molecular weight distribution curve, i.e. the appearanceof the graph of the polymer weight fraction as function of its molecularweight, of such a multimodal polyethylene will show two or more maximaor at least be distinctly broadened in comparison with the curves forthe individual fractions.

For example, if a polymer is produced in a sequential multistageprocess, utilising reactors coupled in series and using differentconditions in each reactor, the polymer fractions produced in thedifferent reactors will each have their own molecular weightdistribution and weight average molecular weight. When the molecularweight distribution curve of such a polymer is recorded, the individualcurves from these fractions are superimposed into the molecular weightdistribution curve for the total resulting polymer product, usuallyyielding a curve with two or more distinct maxima.

The polyethylene composition preferably has an MFR₅ of 0.15 to 0.5 g/10min, more preferably of 0.2 to 0.4 g/10 min.

The base resin preferably has a density of 940 to 946 kg/m³, morepreferably 941 to 945 kg/m³.

The SHI is the ratio of the viscosity of the polyethylene composition atdifferent shear stresses. In the present invention, the shear stressesat 2.7 kPa and 210 kPa are used for calculating the SHI_((2.7/210))which may serve as a measure of the broadness of the molecular weightdistribution.

The SHI of the polyethylene compositions of the present invention iscomparatively low. This is an indication of a rather narrow molecularweight distribution of the base resin. The SHI of the polyethylenecompositions according to the invention is preferably 10 to 45, morepreferably 15 to 35.

In a preferred embodiment the polyethylene composition further comprisesa nucleating agent. The amount of such a nucleating agent in thepolyethylene composition is preferably 0.01 to 0.5 wt %, furtherpreferred 0.05 to 0.25 wt %.

The nucleating agent may be any compound or mixture of compounds capableof nucleating the crystallization, such as a pigment having a nucleatingeffect or an additive used only for nucleating purposes. Examples of thefirst category of compounds are phthalocyanine blue or green pigments(e.g. PB15:1, PB15:3, PG7), isoindolinone and isoindoline pigments (e.g.PY109, PY110, PO61), benzimidazolone pigments (e.g. PO62, PO72),quinacridone pigments (e.g. PY19), benzimidazolone pigments (e.g. PY180,PY181), quinophthalone pigments (e.g. PY138), chinacridone pigments(e.g. Pigment Violet PV19) and azoheterocyclus pigments (e.g. PO64).

The nucleating agent may also be a polymeric additive, such as a polymerof vinylcyclohexane or 3-methyl-1-butene. In such case, the polymericadditive, which preferably has a melting point above 200° C., may beblended into the bimodal polymer by conventional means in an extruder,or it may be prepolymerized on the catalyst as disclosed e.g. in WO99/24478.

Fraction (A) preferably has a MFR₂ of 10 to 300 g/10 min, morepreferably 20 to 200 g/10 min, still more preferably of 30 to 100 g/10min, most preferably of 45 to 70 g/10 min.

Fraction (A) preferably has a density of 955 to 980 kg/m³, morepreferably 960 to 980 kg/m³, and even more preferably 970 to 980 kg/m³.

Furthermore, fraction (A) preferably is an ethylene homopolymer.

The shear stress η_(2.7 kPa) of the polyethylene composition ispreferably 80 to 230 kPas, more preferably 100 to 210 kPas and stillmore preferably 130 to 200 kPas.

The flexural modulus of the polyethylene composition is preferably 500to 900 MPa, more preferably 700 to 900 MPa.

The weight split in the base resin between fraction (A) and fraction (B)is preferably (30-47): (70-53), more preferably (35-45): (65-55).

Furthermore, the polyethylene composition has a good rapid crackpropagation resistance. A pipe made of the multimodal polyethylenecomposition according to the present invention preferably has a ductilebrittleness temperature (T_(crit.)) of −12° C. or lower, more preferably−15° C. or lower (RCP-S4 value).

Still further, the polyethylene composition has a slow crack propagationresistance of at least 500 h, more preferably of at least 1000 h, stillmore preferably of at least 2000 h, and most preferably of at least 4000h at 5.5 MPa hoop stress and 9.2 bar internal pressure at 80° C.

A pressure pipe made of the multimodal polymer composition according tothe present invention preferably has a design stress rating of at leastMRS8.0, and more preferably of at least MRS10.0.

Preferably, the polyethylene compositions, without Carbon black orfillers, of the present invention fulfil the following relationship:

$\frac{FM}{\ln \left( {\frac{\eta_{747{Pa}}}{{SHI}_{2.7/210}} \times {MFR}_{5}} \right)} \leq 950$

wherein FM denotes the flexural modulus as described above.

The numerator of the above given formula defines the flexibility of thematerial. If the flexibility becomes too high, however, the materialloses its ability to withstand pressure. The denominator defines thepressure resistance of the material. Therefore, the above givenrelationship shows how to find a polyethylene composition which fulfilsboth the demands of flexibility and pressure resistance.

The base resin of the polyethylene composition preferably comprises atleast 0.2 mol %, more preferably at least 0.75 mol %, and still morepreferably at least 0.95 mol % of at least one alpha-olefin comonomer.The amount of comonomer is preferably at most 3.0 mol %, more preferablyat most 2.5 mol %, and still more preferably at most 2.0 mol %.

Fraction (B) of the polyethylene composition preferably comprises atleast 0.3 mol %, more preferably at least 0.6 mol %, and still morepreferably at least 0.8 mol % of at least one alpha-olefin comonomer.The amount of comonomer is preferably at most 6.0 mol %, more preferablyat most 5.0 mol %, and still more preferably at most 4.0 mol %.

As an alpha-olefin comonomer, preferably an alpha-olefin having from 4to 8 carbon atoms is used. Still more preferably an alpha-olefinselected from 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene isused.

Where herein features of fractions (A) and/or (B) of the composition ofthe present invention are given, these values are generally valid forthe cases in which they can be directly measured on the respectivefraction, e.g. when the fraction is separately produced or produced inthe first stage of a multistage process.

However, the base resin may also be and preferably is produced in amultistage process wherein e.g. fractions (A) and (B) are produced insubsequent stages. In such a case, the properties of the fractionsproduced in the second and third step (or further steps) of themultistage process can either be inferred from polymers, which areseparately produced in a single stage by applying identicalpolymerisation conditions (e.g. identical temperature, partial pressuresof the reactants/diluents, suspension medium, reaction time) with regardto the stage of the multistage process in which the fraction isproduced, and by using a catalyst on which no previously producedpolymer is present. Alternatively, the properties of the fractionsproduced in a higher stage of the multistage process may also becalculated, e.g. in accordance with B. Hagström, Conference on PolymerProcessing (The Polymer Processing Society), Extended Abstracts andFinal Programme, Gothenburg, Aug. 19 to 21, 1997, 4:13.

Thus, although not directly measurable on the multistage processproducts, the properties of the fractions produced in higher stages ofsuch a multistage process can be determined by applying either or bothof the above methods. The skilled person will be able to select theappropriate method.

The polyethylene composition according the invention preferably isproduced so that at least one of fractions (A) and (B), preferably (B),is produced in a gas-phase reaction.

Further preferred, one of the fractions (A) and (B) of the polyethylenecomposition, preferably fraction (A), is produced in a slurry reaction,preferably in a loop reactor, and one of the fractions (A) and (B),preferably fraction (B), is produced in a gas-phase reaction.

Further, the polyethylene base resin preferably is produced in amultistage process. Polymer compositions produced in such a process arealso designated as “in-situ”-blends.

A multistage process is defined to be a polymerisation process in whicha polymer comprising two or more fractions is produced by producing eachor at least two polymer fraction(s) in a separate reaction stage,usually with different reaction conditions in each stage, in thepresence of the reaction product of the previous stage which comprises apolymerisation catalyst.

Accordingly, it is preferred that fraction (A) and (B) of thepolyethylene composition are produced in different stages of amultistage process.

Preferably, the multistage process comprises at least one gas phasestage in which, preferably, fraction (B) is produced.

Further preferred, fraction (B) is produced in a subsequent stage in thepresence of fraction (A) which has been produced in a previous stage.

It is previously known to produce multimodal, in particular bimodal,olefin polymers, such as multimodal polyethylene, in a multistageprocess comprising two or more reactors connected in series. As instanceof this prior art, mention may be made of EP 517 868, which is herebyincorporated by way of reference in its entirety, including all itspreferred embodiments as described therein, as a preferred multistageprocess for the production of the polyethylene composition according tothe invention.

Preferably, the main polymerisation stages of the multistage process aresuch as described in EP 517 868, i.e. the production of fractions (A)and (B) is carried out as a combination of slurry polymerisation forfraction (A)/gas-phase polymerisation for fraction (B). The slurrypolymerisation is preferably performed in a so-called loop reactor.Further preferred, the slurry polymerisation stage precedes the gasphase stage.

Optionally and advantageously, the main polymerisation stages may bepreceded by a prepolymerisation, in which case up to 20 wt %, preferably1 to 10 wt %, more preferably 1 to 5 wt %, of the total base resin isproduced. The prepolymer is preferably an ethylene homopolymer (HDPE).At the prepolymerisation, preferably all of the catalyst is charged intoa loop reactor and the prepolymerisation is performed as a slurrypolymerisation. Such a prepolymerisation leads to less fine particlesbeing produced in the following reactors and to a more homogeneousproduct being obtained in the end.

The polymerisation catalysts include coordination catalysts of atransition metal, such as Ziegler-Natta (ZN), metallocenes,non-metallocenes, Cr-catalysts etc. The catalyst may be supported, e.g.with conventional supports including silica, Al-containing supports andmagnesium dichloride based supports. Preferably the catalyst is a ZNcatalyst, more preferably the catalyst is a non-silica supported ZNcatalyst, and most preferably a MgCl₂-based ZN catalyst.

The Ziegler-Natta catalyst further preferably comprises a group 4 (groupnumbering according to new IUPAC system) metal compound, preferablytitanium, magnesium dichloride and aluminium.

The catalyst may be commercially available or be produced in accordanceor analogously to the literature. For the preparation of the preferablecatalyst usable in the invention reference is made to WO2004055068 andWO2004055069 of Borealis and EP 0 810 235. The content of thesedocuments in its entirety is incorporated herein by reference, inparticular concerning the general and all preferred embodiments of thecatalysts described therein as well as the methods for the production ofthe catalysts. Particularly preferred Ziegler-Natta catalysts aredescribed in EP 0 810 235.

The resulting end product consists of an intimate mixture of thepolymers from the reactors, the different molecular-weight-distributioncurves of these polymers together forming amolecular-weight-distribution curve having a broad maximum or severalmaxima, i.e. the end product is a multimodal polymer mixture.

It is preferred that the multimodal base resin of the polyethylenecomposition according to the invention is a bimodal polyethylene mixtureconsisting of fractions (A) and (B), optionally further comprising asmall prepolymerisation fraction in the amount as described above. It isalso preferred that this bimodal polymer mixture has been produced bypolymerisation as described above under different polymerisationconditions in two or more polymerisation reactors connected in series.Owing to the flexibility with respect to reaction conditions thusobtained, it is most preferred that the polymerisation is carried out ina loop reactor/a gas-phase reactor combination.

Preferably, the polymerisation conditions in the preferred two-stagemethod are so chosen that the comparatively low-molecular polymer havingno content of comonomer is produced in one stage, preferably the firststage, owing to a high content of chain-transfer agent (hydrogen gas),whereas the high-molecular polymer having a content of comonomer isproduced in another stage, preferably the second stage. The order ofthese stages may, however, be reversed.

In the preferred embodiment of the polymerisation in a loop reactorfollowed by a gas-phase reactor, the polymerisation temperature in theloop reactor preferably is 85 to 115° C., more preferably is 90 to 105°C., and most preferably is 92 to 100° C., and the temperature in thegas-phase reactor preferably is 70 to 105° C., more preferably is 75 to100° C., and most preferably is 82 to 97° C.

A chain-transfer agent, preferably hydrogen, is added as required to thereactors, and preferably 200 to 800 moles of H₂/kmoles of ethylene areadded to the reactor, when the LMW fraction is produced in this reactor,and 0 to 50 moles of H₂/kmoles of ethylene are added to the gas phasereactor when this reactor is producing the HMW fraction.

The composition of the invention preferably if produced in a processcomprising a compounding step, wherein the composition of the baseresin, i.e. the blend, which is typically obtained as a base resinpowder from the reactor, is extruded in an extruder and then pelletisedto polymer pellets in a manner known in the art.

Optionally, additives or other polymer components can be added to thecomposition during the compounding step in the amount as describedabove. Preferably, the composition of the invention obtained from thereactor is compounded in the extruder together with additives in amanner known in the art.

The extruder may be e.g. any conventionally used extruder.

Furthermore, the present invention relates to an article, preferably apipe comprising a polyethylene composition as described above and to theuse of such a polyethylene composition for the production of an article,preferably a pipe.

EXAMPLES 1. Definitions and Measurement Methods a) Density

Density is measured according to ISO 1183-2. Sample preparation is donein accordance with ISO 1872-2B.

b) Melt Flow Rate/Flow Rate Ratio

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR is an indication of the flowability, andhence the processability, of the polymer. The higher the melt flow rate,the lower the viscosity of the polymer. The MFR is determined at 190° C.and may be determined at different loadings such as 2.16 kg (MFR₂), 5.00kg (MFR₅) or 21.6 kg (MFR₂₁).

The quantity FRR (flow rate ratio) is an indication of molecular weightdistribution and denotes the ratio of flow rates at different loadings.Thus, FRR_(21/5) denotes the value of MFR₂₁/MFR₅.

c) Rheological Parameters

Rheological parameters such as Shear Thinning Index SHI and Viscosityare determined by using a rheometer, preferably a Physica MCR 300Rheometer distributed by Anton Paar GmbH. The definition and measurementconditions are described in detail on page 8 line 29 to page 11, line 25of WO 00/22040.

d) Rapid Crack Propagation

The rapid crack propagation (RCP) resistance of a pipe is determinedaccording to a method called the S4 test (Small Scale Steady State),which has been developed at Imperial College, London, and which isdescribed in ISO 13477:1997 (E).

According to the RCP-S4 test a pipe is tested, which has an axial lengthnot below 7 pipe diameters. The outer diameter of the pipe is about 110mm or greater and its wall thickness about 10 mm or greater. Whendetermining the RCP properties of a pipe in connection with the presentinvention, the outer diameter and the wall thickness have been selectedto be 110 mm and 10 mm, respectively. While the exterior of the pipe isat ambient pressure (atmospheric pressure), the pipe is pressurisedinternally, and the internal pressure in the pipe is kept constant at apressure of 0.5 MPa positive pressure. The pipe and the equipmentsurrounding it are thermostatted to a predetermined temperature. Anumber of discs have been mounted on a shaft inside the pipe to preventdecompression during the tests. A knife projectile is shot, withwell-defined forms, towards the pipe close to its one end in theso-called initiating zone in order to start a rapidly running axialcrack. The initiating zone is provided with an abutment for avoidingunnecessary deformation of the pipe. The test equipment is adjusted insuch a manner that crack initiation takes place in the materialinvolved, and a number of tests are effected at varying temperatures.The axial crack length in the measuring zone, having a total length of4.5 diameters, is measured for each test and is plotted against the settest temperature. If the crack length exceeds 4 diameters, the crack isassessed to propagate. If the pipe passes the test at a giventemperature, the temperature is lowered successively until a temperatureis reached, at which the pipe no longer passes the test, but the crackpropagation exceeds 4 times the pipe diameter. The critical temperature(T_(crit)), i.e. the ductile brittle transition temperature as measuredaccording to ISO 13477:1997 (E) is the lowest temperature at which thepipe passes the test. The lower the critical temperature the better,since it results in an extension of the applicability of the pipe.

e) Constant Tensile Load (CTL)

The slow crack propagation resistance is determined with this test. TheCTL test is done with reference to ISO 6252:1992 (E), with the notchaccording to ASTM 1473, in the following way:

The CTL test is a test for accelerated slow crack growth where theacceleration is maintained by elevated temperature of 60° C. The testingis performed in a surface active solution and the incorporation of anotch both accelerates the time to failure and ensures a plain strain inthe samples.

The stress in the samples was 5.0 MPa (actual stress in the notchedregion). The surfactant used in the test was IGEPAL CO-730 at atemperature of 60° C.

The samples are prepared by pressing a plaque with a total length of 125to 130 mm and a width at its ends of 21±0.5 mm. The plaque then ismilled into the correct dimensions in a fixture on two of the sides witha centre distance of both holders of 90 mm and a hole diameter of 10 mm.The central part of the plaque has a parallel length of 30±0.5 mm, awidth of 9±0.5 mm, and a thickness of 6±0.5 mm.

A front notch of 2.5 mm depth is then cut into the sample with a razorblade fitted into a notching machine (PENT-NOTCHER, Norman Brownengineering), the notching speed is 0.2 mm/min. On the two remainingsides side grooves of 0.8 mm are cut which should be coplanar with thenotch. After making the notches, the sample is conditioned in 23±1° C.and 50% relative humidity for at least 48 h. The samples are thenmounted into a test chamber in which the active solution (10% watersolution IGEPAL CO-730, chemical substance: 2-(4-Nonyl-phenoxy)ethanol,C₁₇H₂₈O₂) is kept. The samples are loaded with a dead weight and at themoment of breakage an automatic timer is shut off.

f) Pressure Testing and Design Stress

The design stress rating is the circumferential stress a pipe isdesigned to withstand for 50 years without failure and is determined fordifferent temperatures in terms of the Minimum Required Strength (MRS)according to ISO/TR 9080. Thus, MRS 8.0 means that the pipe is a pipewithstanding a hoop stress of 8.0 MPa gauge for 50 years at 20° C., andsimilarly MRS10.0 means that the pipe withstands a hoop stress of 10 MPagauge for 50 years at 20° C.

These values are calculated from the results of the pressure testingwhich are carried out according to ISO 1167. Pipes with a diameter of 32mm are tested at different temperatures and inner pressure.

g) Creep Resistance

The short term creep ratio was measured in a four point bending modeaccording to DIN-Certco ZP 14.3.1 (former DIN 54852-Z4) at 1 min and 200h.

h) Flexural Modulus

Flexural modulus was determined according to ISO 178. The test specimenswere 80×10×4.0 mm (length×width×thickness). The length of the spanbetween the supports was 64 mm, the test speed was 2 min/min and theloadcell was 100 N. The equipment used was an Alwetron TCT 25.

2. Polyethylene Compositions

Production of polyethylene composition base resins was performed in amultistage reaction comprising a prepolymerisation in slurry in a 50 dm³loop reactor, followed by transferring the slurry to a 500 dm³ loopreactor wherein polymerisation was continued in slurry to produce thelow molecular weight component, and a second polymerisation in a gasphase reactor in the presence of the product from the second loopreactor to produce the comonomer containing high molecular weightcomponent. The comonomer was 1-butene in all compositions produced.

As a catalyst, Lynx 200 from Engelhard Corporation in Pasadena, USA, wasused.

For the comparative examples, a Ziegler-Natta catalyst in accordancewith Example 1 of EP 0 688 794 has been used.

The nucleating agent used in the Examples is Pigment Cromophtal blue4GNP (phthalocyanine blue).

The polymerisation conditions applied are listed in Table 1.

Examples 1 and 2, showing compositions 1 and 2, respectively, areExamples according to the invention. Example 3 is a comparative Examplewhich shows composition 3. This is a polyethylene composition accordingto the prior art. In all three Examples in the step of prepolymerizationhomopolymers are produced.

TABLE 1 Example 1 2 3 (Comp.) Prepolymerisation Temperature ° C. 60 6040 Pressure bar 63 63 63 MFR₅ g/10 min 3.5 3.5   0.5 SlurryPolymerisation in Loop Reactor Temperature ° C. 85 85 95 Pressure bar 5758 57 C₂ concentration mol % 4.7 4.0   3.0 H₂/C₂ mol/kmol 325 252 502 C₄/C₂ mol/kmol 0 112  0 MFR₂ g/10 min 60 60 300  Density kg/m³ >970959 >970  Gas Phase Polymerisation Temperature ° C. 85 85 85 Pressurebar 20 20 20 C₂ concentration mol % 16 21   4.8 H₂/C₂ mol/kmol 24 38  5.8 C₄/C₂ mol/kmol 82 64 108  JSW CIM90P Extruder Feed kg/h 221 220SEI kWh/t 304 306 Melt temperature ° C. 230 230 Properties of Base resinDensity kg/m³ 941 941 947  Split 2:38:60 2:38:60 1.5:49.5:49(Prepol./loop/gas phase) Properties of Composition MFR₅ g/10 min 0.240.27    0.29 MFR₂₁ g/10 min 5.0 5.3   9.9 Density kg/m³ 942.5 942.6 959 Comonomer content wt % 1.7 1.2   1.1 Flexural modulus MPa 769 743 1050* T_(crit.) (RCP-S4) ° C. −18 −18 −12   Creep modulus (200 h) MPa 283 297SHI_(2.7/210) 23.1 22.3 98 η_(2.7 kPa) kPas 168 191 260  η_(747 Pa) kPas225 289 580  Pressure testing h >2352 >2328 >1000    (80° C., 5.5 MPa)MRS MPa ≧10.0 ≧10.0 ≧10.0 CTL h >1000 >1400 >1500    Cromophtal blue4GNP wt % 0.1 0.1  0 Carbon black wt % 0 0   2.3 *without Carbon black

1. A polyethylene composition comprising a base resin which comprises(a) an ethylene homo- or copolymer fraction (A); and (b) an ethylenehomo- or copolymer fraction (B), wherein (i) fraction (A) has a loweraverage molecular weight than fraction (B); (ii) the base resin has adensity of 940 to 947 kg/m³, determined according to ISO 1183-2; (iii)the polyethylene composition has an MFR₅ of 0.1 to 0.5 g/10 min,determined according to ISO 1133; and (iv) the polyethylene compositionhas an SHI_((2.7/210)) of 10 to
 45. 2. The polyethylene compositionaccording to claim 1 further comprising a nucleating agent.
 3. Thepolyethylene composition according to claim 1, wherein fraction (A) hasa MFR₂ of 10 to 300 g/10 min, determined according to ISO
 1133. 4. Thepolyethylene composition according to claim 3, wherein fraction (A) hasa MFR₂ of 30 to 100 g/10 min, determined according to ISO
 1133. 5. Thepolyethylene composition according to claim 3, wherein fraction (A) hasa density of 955 to 980 kg/m³, determined according to ISO 1183-2. 6.The polyethylene composition according to claim 1, wherein the shearstress of the polyethylene composition η_(2.7 kPa) is 80 to 230 kPas. 7.The polyethylene composition according to claim 1, wherein the flexuralmodulus of the polyethylene composition is 500 to 900 MPa, determinedaccording to ISO
 178. 8. The polyethylene composition according to claim1, wherein the weight split between fraction (A) and fraction (B) is(30-47): (70-53).
 9. The polyethylene composition according to claim 1wherein the rapid crack propagation resistance of a pipe made of thepolyethylene composition as measured in the S4 test results in aT_(crit) of <−12° C., determined according to ISO 13477:1997 (E). 10.The polyethylene composition according to claim 1 wherein the slow crackpropagation resistance of a pipe made of the polyethylene composition asmeasured according to ISO 6252 (CTL) with a notch according to ASTM 1473is at least 500 h.
 11. The polyethylene composition according to claim 1wherein the base resin comprises 0.2 to 3.0 mol % of at least onealpha-olefin comonomer.
 12. The polyethylene composition according toclaim 1 wherein fraction (B) comprises 0.3 to 6.0 mol % of at least onealpha-olefin comonomer.
 13. An article comprising a polyethylenecomposition according to claim
 1. 14. The article according to claim 13wherein the article is a pipe.
 15. The pipe according to claim 14 whichfulfils the MRS10.0 requirement according to ISO
 9080. 16. (canceled)17. (canceled)